332 



METAZOAN PHYLA 



(Fig. 229). If these tubules belong to metameres located far forward 

 in the body, the organ is called a pronephros, or head kidney; such 

 tubules open freely into the coelom by a ciliated funnel and take the 

 waste from it (Fig. 230). The vascular organ which passes the excretions 

 from the blood into the coelom is known as a glomus. If the metameres 

 represented are farther back, the organ is called a mesonephros; each 



Pronephric 

 tubule 



Cavity 

 of myo- 

 tome 



Pronephric 

 duct 



Glomus 



Coelom 



Mesonephr/c 

 cfucf 



Mesonephnc 

 tubu/e 



6lof77eru/us 



f^ephrostotne 



Coe/om 



B 



Glomeru/us 



Ureter 



copsu/e 



Fig. 230. — Diagrams to show relations of kidney tubules. A, pronephros; 5, mesonephros; 



C, metanephros. 



mesonephric tubule, while still opening into the coelom, takes the 

 excretions from a knot of capillaries known as a glomerulus. The third 

 type, or metanephros, originates still farther posteriorly; the metanephric 

 tubules do not communicate with the coelom, but each ends in a cuplike 

 cavity inclosing a glomerulus. These appear to form both a phylogenetic 

 and an ontogenetic series. The pronephros is functional only in the 

 lowest group of vertebrates; although it appears early in the embryological 

 development of all higher forms, only vestiges remain in the adults 

 of the highest. The mesonephros is functional up to and including the 



